Method of vegetable tanning completely delimed hides



United States Patent METHOD OF VEGETABLE TANNING (M- PLETELY DELINIED HIDES Raymond L. Moore, East Aurora, and Harland T. Markley and Joseph J. Baker, Buffalo, N. Y.

No Drawing. Application October 5, 1954, Serial No. 460,502

6 Claims. (Cl. 8-94.17)

The present invention relates to an improved process for vegetable tanning of leather.

This application is a continuation-in-part of Serial Number 375,553, filed August 20, 1953, now abandoned.

Procedures for the vegetable tanning of leather, particularly heavy leather; that is leather to be used for the soles of shoes, as belting, as harnesses, straps, etc., have been carried outfor a long time. The hide or skin is generally received at the tannery in a dry condition, the hide having been either sun-dried or treated with salt in order to prevent putrefication. At the present time, the green hide is salted, packed and stored in a cool place for about 30 days; this green, salted hide is the product received almost exclusively at the tannery today. This hide or skin is preliminarily treated to obtain the wanted portions thereof.

The hide or skin normally is considered as composed of three distinct sections; namely, the epidermiswhich is generally a thin outer layer of epithelial cells; a relatively thick layer immediately thereunder-the wanted portioncalled the derma, and a subcutaneous layer which is generally adipose tissue and is commonly called flesh.

As the tanner is interested in preparing the derma, both the epidermis and the fatty tissue facing the derma are removed prior to tanning.

Generally the hide, as received at the tannery, is prepared by soaking until it is plumped and relatively pliable. By soaking and washing the hide, the large amount of unwanted soluble material is removed, as well as insoluble foreign material which may adhere to the hide.

In general, the hide is next subjected to a depilation process wherein the hair may either be scraped from the hide or removed by other mechanical means, the last remaining traces of hair being removed at the time of the removal of the epidermis. Thereafter, the hide is limed. This operation comprises immersion of the hide in a lime slurry or coating the skin therewith.- This eifects partial solution of the epidermis and loosens any hair and hair roots s-o-they can be removed from the hair I follicles. The epidermis is then removed and the hide is then generally beamed over the flesher beam to remove the adhering fatty matter and excess tissue adhering to the underside of the skin,

The wanted portion of the hidethe derma-having thus been obtained, the excess lime was washed out of the derma, the so preparedhide treated with a bate enzyme to further prepare the hide which was then subjected to treatment, while in the alkaline condition, with acid solutions of vegetable tanning extract. Generally, the derma was immersed in a weak acidic tanning extract of light concentration and soaked for a considerable period of time, followed by immersion in liquors of greater concentration of tannin and acid. Frequently the tanning was completed by drumming the derma in warm tannin extracts of high concentration.

Although vegetable tanning is extremely desirable, particularly in the case of the production of so-called heavy leathers, it is a long and laborious procedure involving 40 to days or more treatment, and involving very high labor and equipment costs due to changing the liquor concentration and movement of the stock either by hand or mechanically into successively stronger and more acid liquors either in vats or tanning drums.

in vegetable tanning, as heretofore carried out, considerable loss of tanning liquor was always a concomitant of vegetable tanning since much of the weak tanning liquors were sent to the sewers and a considerable quantity of the vegetable tanning material was lost as its effectiveness was reduced by oxidation and heat. Furthermore, the hides were generally washed between the treatments of tanning liquor of different concentrations, which washing was an expensive operation and which washing removed tannin from the hide, which tanning was in such low concentration as not to be economically recoverable and so was lost when the wash water was ditched to the sewer.

In the usual vegetable tanning operation, the tanned leather was normally subjected to a so-called bleaching operation in order to obtain leather of an even and level color.

As the tanning operation was normally performed on the acid side, calcium ions could produce insoluble calcium tannin compounds which were not particularly desirable and which resulted in further loss of tanning material. Vegetable tanning, as heretofore performed, was a high cost operation due to excessive loss of material and due to the large amount of hand labor required.

It is an object of the present invention to provide a method for the vegetable tanning of leather in periods of time of much shorter duration than heretofore required.

It is also an object of the invention to reduce the amount of costly hand labor required heretofore in tanning leather.

It is also an object of the present invention to eliminate the necessity of gradually building up the concentration of tanning liquors as the tanning operation proceeds. A corollary advantage resides in the retention within the system of substantially all of the tanning material initially employed, with the exception of a small mechanical loss.

It is also an object of the invention to provide a procedure wherein there is substantially no loss of tanning material.

The invention further provides the advantage that it is unnecessary to subject the vats and tanks employed to the frequent cleanings heretofore found necessary to eliminate sediment.

It is a further object of the invention to provide a vegetable tanning procedure wherein it is unnecessary to subject the tanned hides to wheel extracting, washing, and bleaching since the hides tanned in accordance with this invention are of an even and level color.

It is an additional object of the invention to provide a vegetable tanning procedure wherein the conditions inducing fermentation in the tanning liquor are eliminated and wherein there is little tendency to lose tanning by the formation of insoluble calcium tannin compounds. Furthermore, as the tanning operation of the present invention is performed at conventional temperatures, there is little loss of tannin from the liquors due to heating.

It is an object of the invention, therefore, to provide a method for the vegetable tanning of leather wherein there is produced, from each pounds of tannin, from 3 to 5 pounds of leather in excess of that produced by the usual processes heretofore employed.

In accordance with the procedures of the present invention for the rapid tanning of leather, the hides are prepared by processes known to be conventional in the usual beam house procedures; that is to say, until the hides have been depilated, fleshed and beamed. However, substantially all the lime is removed from the hide before continuous use of the tannin liquor-with merely a small make-up added for each batch of hides; all the tannin ultimately combines with the hide.

The fleshed and beamed hides are then soaked in water at room temperature, generally overnight. In normal plant operation performed in accordance with the present invention, 2,000 to 3,000 pounds of hides (white weight, i. e., plumped-derma) are placed in 'a vat, as for instance a conventional paddle vat, and covered with from 2,000 to 3,000 gallons of water.- By morning the pH of the water has risen, as the lime is gradually extracted and a finalpH is obtained of from 9 to 10. The paddle wheel is then started and dilute sulfuric acid gradually added, generally in small increments, over a period of /2 hour. After the addition of from 6 to l0.pounds of 66 Baum sulfuric acid has been added, the pH will have been lowered to the range pH 5 to pH 6 and the hides will have been partially delimed and be in contact with a solution in which the pH persists 'inthe range of 5 to 6 as stated.

The partially delimed hides are then removed from the paddle vat and transferred to the bate paddle vat. The charge of hides, as above given, is'covered with 2,000 to 3,000 gallons of water, the temperature of which is slightly above room temperature and in theneighborhood of 85 to 95 F. To this water there has been added of the liquor remains constant in that range for 2 hours, thereby producing" in the hide itself a pH of 5 m6, 'i. e., the hide is alkaline with respect to its iso-electric point.

As mentioned above, the hides, before entering the tannin liquor, are freed of lime. Although the pH adjustment will, for purposes of economy, be made by addition of dilute sulfuric acid, other acids may be employed for the purpose, as for instance acetic acid, lactic acid, formic acid or a mineral acid other than sulfuric acid.

from /2 to 1%, based on original white weight of hides,

approximately pH 4.9, and the hides are maintained in' this liquor for about /2 hour. The hating operation produces an emulsification of any natural fat which remains in the derma and effects a slight hydrolysis of the collagen fibers thereof to obtain a cleaner grain and to produce a more elastic and pliable end product. The hating operation also assists in eliminating further quantities of unwanted lime.

The depilated and partially delimed hides are then removed from the hate vat and suspended in water acidified with sulfuric acid to a pH of 5 to 6 and allowed to re main therein at a temperature approximating room temperature, overnight, whereupon the. pH of the water is again adjusted by the addition of sulfuric acid to the range pH 5 to 6 and allowed to hang for an additional 2 hours and, if after that time the 'pH of the surrounding acid liquor has not changed from the pH range 5 to 6, the great majority of lime is removed from the hides which are now substantially lime free, whereupon the hides or sides are transferred to the tanning operation proper of the present invention, which comprises a loading of the derma with tannin and the setting .or fixing of the tannin in the derma.

If, however, the pH of the surrounding liquor has increased to above pH 6, the solution is acidified by the addition of an amount of acid sufiicient to bring the acidity of the liquor to the range 5 to 6 and the hides left for an additional 2 hours. When the pH of the solution, after 2 hours, remains constant in the range pH 5 to 6, then the hides have been delimed in accordance with this invention, and the pH of :the hides themselves will persist in that pH range and are prepared for the tanning treatment per sc. In other words, the pH of the surrounding liquor is constantly adjusted by acid addition to bring the pH thereof to pH 5 to 6 until such time as the pH hereinafter.

Such removal of lime permits the continuous use of tannin liquor as the latter, after a few packs have been put through, becomes stabilized as to total solids, tannin, purity, and soluble ash content. That is, nothing objectionable accumulates in the liquor that is objectionable from the point of view of tannin absorption by the hide and, in fact, all tannin used in the process is so absorbed except for a slight mechanical loss.

The hides or sides are then transferred to the vegetable tannage vat proper, which vat may be preferably of the rocker type. The vegetabletanning solution in the vat may be any of the suitable'vegetabletanning extracts such as mimosa or golden wattle (Acacia pycnantha), mallet bark (Eucalyptus occidentalis), larch (La'rix em'opaea),

hemlock (Abies camz'densis'), :gambier (Nauclea gambir),

oak bark (Qzzercus'robus), myrobalans (Terminalia chebula), sumach (Rims cariz'zria), galls (Quercus infectoria), divi-divi (Caesalpinia 'co'riaria), valonia (Quercus aegzlops), and others.

For purposes of illustrating the present invention, wattle extract will be used in the illustrations as the vegetable tanning material to be employed and upon hides of cows and steers, in the manufacture of heavy leather.

The above rocker vatis filled with 75 to 'barkometer solid wattle extract liquor at room temperature, i. e., a temperatureof 70'to 75 F. In commercial operation, from 1000 to 1200 pounds of solid wattle extract is dissolved in gallons of $2115 liquor from a previous pack and this 'is added to the'rocker vat to bring'the. volume of liquor therein to approximately 2800 gallons and to produce a barkometer reading in therange specified. In general, a vat of this volume will accommodate a pack of stock of 80 sides. Prior to the introduction of the stock the wattle extract liquor is adjusted to a pH which in the tanning industry is considered to be alkaline; namely, from a pH 6 to pH 8, a pH of 1 to 3 pH units above the iso-electric point of the hide. .This alkalinity is preferably obtained by the addition of a fixed alkali which alkali, by reason of its cheapness, is preferably caustic soda. It will be found, in general, that from 25 to 40 pounds caustic sode, assaying 76% Nazo, will be required to obtain the desired pH of the vegetable extract after each pack of 80 sides has been removed and the make-up, as indicated above. added to the rocker vat.

This stock of 80 sides, as mentioned, is then hung in the heavy wattle liquor, generally from. head to tail, on uniformly spaced L-hooks or other suitable hanging mechanism, on the under side of the rocker frame in the vat until they have absorbed tannin clear to the center of the hide or to a predetermined depth where a split is desired. After soaking in the vegetable tannin liquor for approximately 4 hours, during which time the sides are paddled to keep them separated while in the liquor, the frame is rocked at a relatively slow rate of about 6 revolutions per minute for approximately 7 days and then rocked at a somewhat higher rate for the remainder of the tannage absorption time; that is, 1 to 5 days additional.

The function of the vegetable tanning liquor on the alkaline side of the iso-electric point is to permit tannin entirely to penetrate the hides and subsequently to change the collagen fibers thereof to a water-insoluble nonputrefactive, nonsoluble fiber as more fully described Whereas in the prior methods of tanning, in order to get complete penetration of tannin, the outer surfaces were generally overtanned. Under such conditions, it was necessary to fb-leach, i. e., use alkali to remove the excess tannin to obtain a flexible grain which would not crack, thereby reducing the wear resistance" of the leather.

After being in contact with the vegetable tanning liquor from 8 to 17 days, in order to permit excess tannin to diffuse into the hide, the hides or sides are removed from the rocker vat and passed through a wringer to remove such excess liquor. As removed from the vat, the hides are plumped and very pliable, and the wringing operation removed approximately 17% of the weight of the plumped tannin-containing stock as it comes from the tanning liquor. This liquor is employed to dissolve the wattle extract used in bringing the tanning liquor in the vat up to strength after each pack has been removed, as previously stated.

From the wringers, and without washing, the wrung hides are loaded into a conventional oiling drum where the hides are first neutralized by the addition of an acidic material. In order that the hide be not plumped unduly with water in the presence of the fixing acidic material, the neutralizing is preferably performed with an acid in the presence of a buffering salt and to this end there is employed preferably sodium acid sulfate and oxalic acid to reduce the pH of the hide-tannin complex to below the iso-electric point, i. e., under pH 4.9, or a pH of 3 to 4. Tannin actively combines with the collagen at a pH below the iso-electric point of the hide. The lower the pH the more vigorous is this action.

Although the niter cake, by reason of its effectiveness and cheapness, is preferred, other buffered acids, such as hydrochloric acid or. acetic acid, with sodium acetate, may be used.

In general, about equal parts by weight of the sodium acid sulfate and oxalic acid are used in this neutralization and acidification although the ratio may vary from 40 to 60%. The oiling drum is started in motion and run for about /2 hour at a rate of speed of about 20 revolutions per minute, which gives sufficient time for these acidifying materials to diffuse into the hides and to set the tannin-collagen complexes formed in the tannage operation. A fundamental change is thus produced in the chemical and physical characteristics of the collagen fibers which are now fixed or set, are non-soluble in water, nonputrefactiveand are tough and pliable; the end product is tanned leather.

The leather is then finished by the addition to the wheel of conventional finishing materials, such as liquid corn sugar or dextrose, mineral oil, sulfonated oils, and the like.

The pH of the leather coming from the wheel taken wet is generally 3.0 to 3.4 pH but may vary from this range. If so, it may be adjusted in the subsequent dry dip or second treatment. The leather is then sent to the dry loft and, when dry, given the usual commercial dry dip and second treatment. The dry dip, in accordance with the present invention, due to the resultant high resistance of the leather so produced to water absorption, requires from 1 /2 to 2 hours immersion, depending upon the thickness of the leather.

The above improved procedure makes possible the production of an increased amount of leather per unit of tanning extract employed, greatly decreases the overall time required to produce leather, eliminates the usual loss of tannin and substantially eliminates the waste disposal problems which have plagued the vegetable tanner.

What is claimed is:

1. The method of tanning which comprises soaking a depilated and partially delimed hide in an acidic aqueous solution of pH 5 to 6 and above the iso-electric point of the hide, said solution being initially free of salts, until the pH of the solution has changed and is above the pH range 5 to 6, then constantly adjusting the pH of said solution by adding acid thereto to maintain the pH thereof inv the range pH 5 to 6 and above the iso electric point of the hide until the pH of the solution; in contact with the hides remains constant for two hours in the pH range 5 to 6, thereby producing in the hide itself a persistent pH in the range 5 to 6 and above the iso-electric point of the hide, the hide being substantially lime free, and then adding tannin to the hide by soaking the hide in a vegetable tanning extract at room temperature and of a concentration of 75 to barkometer at a pH of about 1 to 3 pH units above the iso-electric point of the hide until the tanning extract has penetrated to the center of the hide and then adding to the hide in the absence of excess tanning extract above that required to tan the hide an amount of oxalic acid and sodium acid sulfate sufficient to change the pH thereof to pH 3 to pH 3.4 until the tanning extract is fixed by the hide to produce leather.

2. The method of tanning which comprises soaking a depilated and partially delimed hide in an acidic aqueous solution of pH 5 to 6 and above the iso-electric point of the hide, said solution being initially free of salts, until the pH of the solution has changed and is above the pH range 5 to 6, then constantly adjusting the pH of said solution by adding acid thereto to maintain the pH thereof in the range pH 5 to 6 and above the isoelectric point of the hide until the pH of the solution in contact with the hides remains constant for two hours in the pH range 5 to 6, thereby producing in the hide itself a persistent pH in the range 5 to 6 and above the iso-electric point of the hide, the hide being substantially lime free and then adding tannin to the hide by soaking the hide in a vegetable tanning extract at room tem-' perature and of a concentration of 75 to 85 barkometer at a pH of about 1 to 3 pH units above the iso-electric point of the hide until the tannin extract has penetrated to the center of the hide, expressing excess tanning ex-i tract from the hide above that required to tan the hide, and then adding an amount of oxalic acid and sodium acid sulfate to the tanning extract impregnated hide sufficient to change the pH thereof to pH'3 to pH 3.4 until the tanning extract is fixed by the hide to produce leather.

3. The method of tanning which comprises soaking a depilated and partially delimed hide in an acidic aqueous solution of pH 5 to 6 and above the iso-electric point of the hide, said solution being initially free of salts, until the pH of the solution has changed and is above the pH range 5 to 6, then constantly adjusting the pH of said solution by adding acid thereto to maintain the pH thereof in the range pH 5 to 6 and above the iso-electric point of the hide until the pH of the solution in contact with the hides remains constant for two hours in the pH range 5 to 6, thereby producing in the hide itself a persistent pH in the range 5 to 6 and above the iso-electric point of the hide, the hide being substantially lime free and then adding tannin to the hide by soaking the hide in a vegetable tanning extract at room temperature and of a concentration of 75 to 85 barkometer at a pH of about 1 to 3 pH units above the iso-electric point of the hide for from 8 to 17 days and then adding to the hide in the absence of excess tanning extract above that required to tan the hide an amount of oxalic acid and sodium acid sulfate suificient to change the pH thereof to pH 3 to pH 3.4 until the tanning extract is fixed by the hide to produce leather.

4. The method of tanning which comprises soaking a depilated and partially delimed hide in an acidic aqueous solution of pH 5 to 6 and above the iso-electric point of the hide, said solution being initially free of salts, until the pH of the solution has changed and is above the pH range 5 to 6, then constantly adjusting the pH of said solution by adding acid thereto to maintain the pH thereof in the range pH 5 to 6 and above the iso-electric point of the hide until the pH of the solution in contact with the hides remains constant for two hours in the pH range 5 to -6, thereby producing in the hide itself a persistent pH in the ranget to 6 and-above the isoelectric pointofthe hide, thehide being substantiallylime free and then adding tannin to the hide by soaking the hide in a wattle tanning extract at room temperature and'of a concentration of 75 to 85 barkorneter at a pH of about 1 to 3 pH units .above the iso-electric point of the hide until the tanning extract has penetrated to the center of the hide and then adding to the hide in the absence of excess tanning extract above that required to tan the hide an amount of oxalic acid and sodium acid sulfate sufiicient to change the pH thereof to a pH 3 to pH 3.4 until the tanning extract is fixed by the hide to produce leather.

5. The method of tanning which comprises soaking a depilated and partially delimed hide in an acidic aqueous solution of pH 5 m6 and above the iso electric point of the hide, said solution being initially free of salts, until the QPHOf thesolution has changed and is above the pH range 5 to 6, then constantly adjusting the pH of said solution by adding acid thereto to maintain the pH tl16I'6-,

oflin the range pHS to 6 and above the iso-electric point of the hide until the pH of the solution in contact with the hides remains constant for two hours in the pH range 5 to 6, thereby producing in the hide itself a persistent pH in'the range 5 to 6 and above the iso-electric point of the hide, the hide being substantially lime free and then adding tatmin to the hide by soaking the hide in a vegetable tanning extract at room temperature and of a concentration of 75 to 85 barkometer at a pH of about 1 to 3 pH units above the iso-electric point of the hide until the tanning extract has penetrated to a predetermined depth in the hide and then neutralizing the alkaline condition of the hide in the absence of excess tanning extract above that required to tan the hide witha buflered acid to lower the pH- of the hide below its iso-electric point and until the tanning extract is fixed by the hide to produce leather.

j 6. The methodJof tanning which comprises soaking a depilated and partially delimed hide in an acidic aqueous solutionof pH -5Yto 6 and above theiso-electric point of the hide, said solution being initially free of salts, until the pH of the solution has changed and is above the pHrange 5 to .6, then constantly adjusting the .pHof said, solution by addingacid thereto to maintain the pH thereof in the range pH 5 to 6 and above the iso-electric point of the hide until the pH of the solution in contact with the hides remains constant for two hours in the pH range 5 to 6, thereby producing in the hide itself a persistent pH-in the range 5 to 6 and above the iso-electric point of the hide, the hide being substantially lime free and then adding tannin to the hide by soaking the hide in a absence .of excess tanning extract above that required to tan the hide with .a buffered acid to lower the pH of the hide below its iso-electric point and until the tanning extract is fixed by the hide to produce leather.

References Cited in the file of this patent UNITED STATES PATENTS I =Peace Feb. 28, 1933 FOREIGN PATENTS 502,408 Great Britain Dec. 20, 1.928

Great Britain Feb. 3, 1936 

5. THE METHOD OF TANNING WHICH COMPRISES SOAKING A DEPILATED AND PARTIALLY DELIMED HIDE IN AN ACIDIC AQUEOUS SOLUTION OF PH 5 TO 6 AND ABOVE THE ISO-ELECTRIC POINT OF THE HIDE, SAID SOLUTION BEING INITIALLY FREE OF SALTS, UNTIL THE PH OF THE SOLUTION HAS CHANGED AND IS ABOVE THE PH RANGE 5 TO 6, THEN CONSTANTLY ADJUSTING THE PH OF SAID SOLUTION BY ADDING ACID THERETO TO MAINTAIN THE PH THEREOF IN THE RANGE PH 5 TO 6 AND ABOVE THE ISO-ELECTRIC POINT OF THE HIDE UNTIL THE PH OF THE SOLUTION IN CONTACT WITH THE HIDES REMAINS CONSTANT FOR TWO HOURS IN THE PH RANGE 5 TO 6, THEREBY PRODUCING IN THE HIDE ITSELF A PERSISTENT PH IN THE RANGE 5 TO 6 AND ABOVE THE ISO-ELECTRIC POINT OF THE HIDE, THE HIDE BEING SUBSTANTIALLY LIME FREE AND THEN ADDING TANNIN TO THE HIDE BY SOAKING THE HIDE IN A VEGETABLE TANNING EXTRACT AT ROOM TEMPERATURE AND OF A CONCENTRATION OF 75 TO 85 BARKOMETER AT A PH OF ABOUT 1 TO 3 PH UNITS ABOVE THE ISO-ELECTRIC POINT OF THE HIDE UNTIL THE TANNING EXTRACT HAS PENETRATED TO A PREDETERMINED DEPTH IN THE HIDE AND THEN NEUTRALIZING THE ALKALINE CONDITION OF THE HIDE IN THE ABSENCE OF EXCESS TANNING EXTRACT ABOVE THAT REQUIRED TO TAN THE HIDE WITH A BUFFERED ACID TO LOWER THE PH OF THE HIDE BELOW ITS ISO-ELECTRIC POINT AND UNTIL THE TANNING EXTRACTIS FIXED BY THE HIDE TO PRODUCE LEATHER. 